1. Field of Invention
This invention relates to physical-vapor deposition (PVD) and methods for depositing magnetic materials with planar magnetron sputtering system
2. Sputtering Deposion of Prior Art
Sputtering is a method of physical-vapor deposition (PVD) that involves the removal of material from a solid cathode by bombarding it with positive ions from the discharge of a rare gas such as argon (Ar). The cathode can be made of a metal or an insulator and in contrast to thermal evaporation, complex compounds such as high-temperature superconductor (HTS) materials can be sputtered with less chemical-composition change. Sputtering is often done in the presence of a reactive gas, such as oxygen or nitrogen, to control or modify the properties of the deposited film. The following are some of the advantages of the sputtering method:
Environmentally benign process systems compared with chemical processes
Choice of a wide range of deposition rates for the best growth conditions
Control of a wide range of oxygen or nitrogen levels in the dielectric films
Use of oxide or non-oxide targets (reactive sputtering deposition)
Use of single or multi co-sputtering processes
Growth of c-axis oriented layers on amorphous substrates
Growth of not only c-axis but also a-axis oriented layers on a single-crystalline substrate
The sputtering deposition system provides high-density nucleation, which has not only a c-axis but also an a-axis orientation on single-crystalline substrates. This process is ideal for the first, or nucleation step; however, it fails to make a single crystal because of the difficulty to maintain thermal-equilibrium growth-conditions at higher temperatures necessary to grow a single crystal. This is described in Onishi et al, xe2x80x9cChemical Vapor Deposition of Single-Crystalline ZnO Film with Smooth Surface on Intermediately Sputtered ZnO Thin Film on Sapphirexe2x80x9d.
The planar magnetron system is simple and provides high deposition rates from a simple flat target. The conventional system has permanent magnets behind the target that provide strong magnetic fields on the target. The magnetic fields confine high-density plasma to the target. The plasma on the target enhances the deposition rate dramatically. If it is a magnetic target, however, magnetic properties bypass the magnetic fields. Hence magnetic fields on the target will be greatly reduced. Magnetic materials cannot be deposited effectively with a conventional planar magnetron system.
Magnetron systems are very good for Physical Vapor Deposition (PVD) systems as a material source to be deposited because deposition rates are high and excess electron bombardment of the substrate is reduced. This is described in Onishi et al, xe2x80x9cTransparent and Highly Oriented ZnO Films Grown at Low Temperature by Sputtering With a Modified Sputter Gunxe2x80x9d. The planar magnetron generates magnetic fields through the target. The strong magnetic field on the target confines the high density plasma causing target erosion. The conventional target will become thinner as erosion advances and magnetic fields on the eroded areas become stronger. The erosion profiles become deeper narrow rings. The stronger magnetic field accelerates erosion. It creates a narrow, deeper channel. This effect leads to a shorter target life and affects the uniformity of the deposited film on the substrate. The target utilization rate is also lower. To partially solve this problem, costly rotating magnets are required. The rotating magnets act as a magnetic break. This requires a significantly high power motor and excess heat generated on the target becomes a problem.
1. The new magnetron-sputtering target has the magnets on the substrate-facing surface of the magnetic target rather than behind the target so that strong magnetic fields can be applied to the target surface with smaller magnets.
2. Magnets to be exposed in the plasma may be coated with proper magnetic and/or non-magnetic materials by plating them on the magnet surfaces. This practice is already in use with the conventional magnetron systems to prevent corrosion.
3. The required magnets are very small and provide stronger magnetic flux on the target. Magnetic circuits can be designed more precisely for these frontmounted magnets than for those on the back of the target.
4. Better magnetic circuit design eliminates the need for rotated magnetic fields and provides a more uniform deposit.
The permanent magnets will be placed on the magnetic target rather than behind the target. The major erosion area is between the opposite polarities permanent magnets that are on the magnetic target. The permanent magnets erode very little, but they may be coated with suitable materials to prevent cross contamination. In this configuration, permanent magnet strips or rings form magnetic fields directly on the target, where as the conventional planar magnetron generates magnetic fields through the target. My innovative magnetic circuit design does not limit the thickness of the target and magnetic distribution is far better than that of the conventional design. The required permanent magnets are smaller and much less expensive.
FIG. 1 shows a typical planar magnetron in a vacuum chamber incorporating this new target design. All permanent magnets have a polarization from top to bottom and the target provides a common base for the magnetic circuits set up by the permanent magnets. Strong magnetic fields between opposite permanent-magnet polarities trap and confine the high-density plasma. This high-density plasma on the target enhances target erosion and as erosion advances, the magnetic fields tend to be weaker. This results in wider erosion profiles. Since the magnetic circuits are directly exposed rather than through the thick target, smaller permanent magnets can be used. The smaller magnets make it possible to achieve more efficient erosion patterns. This leads to a more uniform sputtering source without rotating magnets or magnetic fields. Although the rotating magnetic assembly improves deposition uniformity, the rotation reduces the magnetic field on the target, generates more heat due to magnetic break effect, hinders ideal electrical feeding system, and triggers plasma instability, including abnormal arc discharges.